The East of England Ambulance service said those critically injured have been taken to Broomfield Hospital while the other eight have been taken to Colchester General Hospital.

An EEAST spokesman said: "A paramedic response car was at the scene in just over 3 minutes after several 999 calls were received at 8.27am.

"A man, believed to be in his 70s, was airlifted to Broomfield Hospital in a life-threatening condition after being anaesthetised on scene by air ambulance paramedics.

"A woman, believed to be in her 50s, was taken to Broomfield with severe burns.

"The other casualties were taken with serious and minor injuries to CGH. Injuries include burns, and trauma injuries such as broken legs.

"Five ambulances, several ambulance incident officers, the hazardous area response team (HART) and two air ambulances attended the scene and treated casualties on scene before they were taken to hospital."

Resident Stuart Coe told Sky News: "We were sitting there having breakfast and the next thing we heard this almighty bang and literally everything shook in the house - the conservatory shook, it felt like the roof was shaking.

"Obviously, the kids were upset and crying, we didn't know what was going on. And then we started fearing there had been an explosion and one of the neighbours believes she saw British Gas in the area yesterday.

"Within minutes of the explosion, we started hearing sirens coming down the road.

"It's like a bomb going off... It was a pretty nasty event."

Cloes Lane resident Gwen Mitchell, 70, who lives about 30 yards from the blast scene, said the house caught in the explosion was a semi-detached property.

Mrs Mitchell, a Conservative councillor on Tendring District Council, said: "I didn't know anything about it until I had got home.

"It's about 12 to 14 houses up the road from me on the opposite side to me. There was a very strong acidy, smoky smell in the air.

"There are lots and lots of police, ambulance and fire engines in the road, and it looks like one of the houses has gone.

"My heart has been doing flips ever since I heard the news. You don't expect this to happen so close to your own home.

"I thank God that it's not me involved and I dearly hope that no one is badly injured. There is insulation being blown up the road, which I presume has come from the house, and we have had people from the gas and electricity boards here."

One neighbour told the Clacton and Frinton Gazette: "The house five doors from us just went bang.

"Our windows have been put through."

An Essex Police spokesman said: "Several houses have been damaged. There have been reports of casualties.

"The immediate area is currently being evacuated. People are advised to avoid the area while emergency services deal with the scene."

Teresa Reed, a lollipop lady from nearby Towse Close, said she heard a "huge bang" as she walked to work.

She added: "Apparently people heard it in the middle of town but I had no idea what it was. It shook the ground and knocked people out of bed.

"I walked past a minute before the blast - a moment later and I could have been hit.

"I know one lady who lived close by so I'm just praying she's okay."

Essex County Fire and Rescue Service said a house at the centre of the blast was on fire, adding that there were multiple casualties.

The incident commander reported two semi-detached houses have been affected by the blast, causing a gas-fuelled fire, which is being allowed to burn under control.

A spokesman said: "Numerous other properties have been affected by flying debris and are being evacuated.

"Firefighters are searching the rubble and dealing with multiple casualties with various injuries. There are still people unaccounted for."

Assistant chief fire officer Paul Hill said the probable cause of the fire was a domestic gas fault.

Locals took to Twitter to share their shock.

Jason Leonard wrote: "Okay so that massive bang was a gas leak & a house blowing up? Pretty standard for a Clacton morning...

"Never heard a bang so loud in my life!"

Andrew Woodfield posted: "My whole house just shook like an explosion gone off in Clacton or something! Anyone know anything??"

Scott Chamberlain tweeted: "What the hell was that?

"Sounded like an explosion somewhere, the whole house shook and now I can hear a lot of sirens."